Use of chemical weapons is punishable under established international law and norms.

Therefore, the United States may punish Syria.

This syllogism has two premises that have factual, legal, and moral merit. As such, the first justification component—factual merit—does not seem to be too controversial, notwithstanding the Assad government’s less-than-convincing protestations.
However, the shift to the conclusion of the syllogism is more controversial: factual merit neither logically requires nor conclusively justifies punishment of Syria by the United States. As such, it implicates the second jus ad bellum war justification component: legitimate authority to be the enforcer. The problem for the Administration has been trying to justify unilateral action, or action by a modest coalition of the willing. Authority for the United States to act in the mode of the enforcer in Syria implicates international and domestic legitimacy questions.
International legal and diplomatic principles recognize unilateral military action taken in the right of self-defense. As such, we often see military strike justification debates follow these lines. Such debates also become somewhat tortured as we move from traditional response-to-attack scenarios into preemptive strikes along the lines of President George W. Bush’s pre-Iraq invasion justification. However, here, we have no self-defense argument being meaningfully advanced..."